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Bill
Never being able to remember the definition of symmedian point for longer than 30 minutes, I Googled it, and that led me to your tutorial https://www.youtube.com/watch?v=pzDyi7ANoVA .  It extends well from a number of connecting points from ‘A’ levels, which in my case was a long time ago.  On the whole, I think I preferred the geometrical solution although it does explain why all the papers I attempt to read in the Journal of Navigation or listen to at conferences are brim full of rows of matrices.

I was pleased that you stressed that your position lines need to have equal bands of error and that your solution is not necessarily what a ‘rule of thumb’ kind of navigator might have used.  A useful thought which you could have left for your potential students, who seemed unusually attentive, to go away and think about is how they might have dealt with the fact they might also have a lovingly worked out DR position to include in the casserole.  Should they ignore it, or should they seek a further MPP?  DaveP